When a drop of coffee dries on a surface, its edges are held in place, preventing the drop from shrinking as it dries. Instead, it must flatten out while maintaining its original circumference. This flattening pushes water – and the particles suspended in it – to the edges so that by the time the drop dries, most of the particles have bunched up to form a ring that's annoyingly hard to remove.

Yunker and his team found that if the particles were spherical, they detached easily from the boundary of the drop and accumulated at the edges. However if they were torpedo-shaped, they would be distributed evenly through the drop as it dried. The discovery could be applied to drying ink or paint: adding elongated particles could help it dry more evenly.

Good luck there on converting your torpedo-shaped coffee particles to spherical ones using your at-home kit.

Anon
on September 2, 2011 2:17 PM

This article is clearly just using coffee as an example, seeing as it's something that most people would witness on a daily basis. If you commenters had managed to finish reading the article, it mentions that it could be applied to other liquids to improve upon them.

Robert D. Blackledge
on September 5, 2011 8:57 PM

Another but totally different way of defeating the coffee ring effect was covered in: Ikeda, Masahiko and Uchihara, Hiroshi, "Liquid Sample Concentration Technique Using Perfluorated Polymer FILM for Picogram Analysis by FT-IR" Applied Spectroscopy, 46(9), 1431-1434 (1992).

TSL
on September 6, 2011 10:07 AM

Important part of the article -- for those that couldn't make it through the entire 3 paragraphs... "The discovery could be applied to drying ink or paint: adding elongated particles could help it dry more evenly."